Carbohydrate composition and construction

Carbohydrates contain the elements carbon, hydrogen and oxygen. Some carbohydrates are small molecules, while others are large molecules built from many smaller units.

Glucose is a small carbohydrate molecule. Many glucose molecules can join together to form a large carbohydrate molecule. Starch, glycogen and cellulose are all large carbohydrates constructed from glucose units. Each consists of many glucose molecules joined together.

Protein composition and construction

Proteins are large molecules made from smaller building blocks called amino acids. They contain the elements carbon, hydrogen, oxygen and nitrogen.

A protein is constructed by joining amino acids together in a chain. The amino acids have a particular sequence: the order in which they occur along the chain. Different proteins can contain different sequences of amino acids, so their chains can be built in different ways.

Iodine test for starch

Iodine solution can be used to find out whether a sample contains starch. Add iodine solution to the sample and observe its colour:

• Blue-black: starch is present. The iodine solution has changed from its original orange-brown colour.
• Orange-brown: starch is absent. The iodine solution has not changed colour.

Benedict’s test for reducing sugars

Benedict’s test shows whether a sample contains reducing sugars. Mix the sample with blue Benedict’s solution, then heat the mixture in a hot water bath.

A positive result is a colour change from blue towards green, yellow, orange or brick-red. The colour reached indicates the amount of reducing sugar present: green indicates a relatively small amount, while orange or brick-red indicates a larger amount.

A negative result remains blue after heating.

Biuret test for proteins

The biuret test detects protein in a sample. Add biuret reagent, which is blue, to the sample and observe the colour of the mixture.

• A change from blue to lilac or purple indicates that protein is present.
• If the mixture remains blue, protein has not been detected.

Ethanol emulsion test for fats and oils

The ethanol emulsion test detects fat or oil in a sample. Fat and oil dissolve in ethanol but do not dissolve in water. When the ethanol mixture is added to water, any fat or oil forms tiny droplets that make the liquid look cloudy.

1. Shake the sample with ethanol.
2. Add the ethanol mixture to water.
3. Observe the appearance of the liquid.

A white, cloudy emulsion indicates that fat or oil is present. If the liquid remains clear, neither fat nor oil has been detected.

DCPIP test for vitamin C

DCPIP is a blue solution used to test for vitamin C. Add the sample to the DCPIP solution and observe its colour.

• Colourless: vitamin C is detected in the sample.
• Blue remains: vitamin C is not detected in the sample.

DNA structure and base pairing

A DNA molecule consists of two strands coiled around each other to form a double helix. Each strand contains chemicals called bases. Bonds between bases on opposite strands hold the two strands together.

The bases pair in a fixed way:

• A (adenine) pairs with T (thymine).
• C (cytosine) pairs with G (guanine).

This fixed matching is called complementary base pairing. The base on one strand determines which base is opposite it on the other strand. For example, if one strand has the sequence A–C–T, the bases opposite it are T–G–A, in that order.